Calibration and operation of the polaris 18-detector CdZnTe array

The Polaris system is an array of 18 position-sensitive detectors, using 6 cm³ pixelated CdZnTe crystals from Redlen Technologies and readout electronics from Gamma-Medica Ideas (GMI). Many detectors received from Redlen Technologies are capable of achieving 1% energy resolution or better at 662 keV for all single-pixel events during room temperature operation. Furthermore, 3D position reconstruction capability enables 4p Compton imaging of gamma rays that undergo multiple interactions within a single detector or between multiple detectors in the array. However, there are many factors that can negatively influence the performance of this array. Measurements at elevated ambient temperatures indicate that the energy resolution degrades rapidly as the detector crystals are operated above 30°C. Furthermore, the electron mobility and trapping change with temperature which degrades the accuracy of the position dependent signal reconstruction parameters. The peak hold circuitry in the GMI ASIC provides a time dependent output, resulting in smaller recorded pulse heights for events with longer delay times between charge collection and system readout. This peak hold drop introduces depth dependent non-linearity for single pixel events and degrades the energy resolution of multiple pixel events. This effect is corrected by measuring the test pulse amplitude as a function of readout delay time. The problem of false triggers must be addressed carefully, as Polaris has 2178 anode and 18 cathode electrodes each independently capable of triggering the system. Collimation studies have been used to evaluate the accuracy of the depth reconstruction techniques and to identify the relative position of each detector crystal in the array, which ultimately affects image reconstruction.